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<p><strong>Probe set data: </strong>The expression data were processed by Laura Saba (UCDHSC). The original CEL files were read into the R environment (Ihaka and Gentleman 1996). Data were processed using the Robust Multichip Average (RMA) method (Irrizary et al. 2003). Values were log2 transformed within the rma function in R. This data set includes further normalization to produce final estimates of expression that can be compared directly to the other transforms.</p>
<p>This includes an initial quantile normalization on the RMA normalized probe set data followed by a transformation to force an array average of 8 units and stabilized standard deviation of 2 units within each array. Please see Bolstad and colleagues (2003) for a helpful comparison of RMA and two other methods of processing Affymetrix array data sets.</p>
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<p>Expression estimates (strain averages) range from a low of about 3.8 for probe set 1457109_x_at to a high of 15 for <em>Gapdh</em> (probe set 1418625_s_at). The mean expression of 8.0 actually represents a relatively low value of expression (roughly 250 on the original scale) because it is the average of all transcripts on the array, including those that are not expressed. Nonetheless, it is possible to obtain good signal down to very low values. For example, probe set 1437432_a_at (<em>Trim12</em>) has an average expression of 4.56 (extremely low), but it still is associated with a strong QTL (LRS of 45) precisely at the location of the parent gene (Chr 7 at 104 Mb). This demonstrates unequivocally that the small strain differences in expression of <em>Trim12</em> measured by probe set 1437432_a_at is not noise but is generated by true allelic differences in <em>Trim12</em> mRNA binding to the arrays.</p>
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